The invention relates to a sputtering system for deposition of materials on wafers and, more particularly, to a system having a perforated cathode accommodating the wafers in the perforations and a heating element in close proximity to the cathode for direct heating of the wafers. The method aspect of the invention relates to forming metal silicide contacts on the wafer by directly heating the wafer, sputter cleaning the contact openings, depositing the metal and in-situ converting the metal to metal silicide.
The present trend in semiconductor technology is toward very large scale integration and ultra large scale integration of devices with very high speed and low power dissipation. To achieve this, the devices are being shrunk in size by, for example, making the vertical junction structure shallower and reducing the horizontal geometry and the electrical resistance associated with the device contacts, interconnection metallization, etc. is being reduced. One extensively used metallurgy for providing both contacts (for example, Ohmic or Schottky barrier contacts which are obtained when a highly or lightly doped region of a silicon substrate are directly contacted, respectively) and interconnections in present circuits is a metal-silicon compound known as a silicide. In this context, silicides such as platinum silicide, palladium silicide, tantalum silicide, titanium silicide, hafnium silicide, zirconium silicide, molybdenum silicide, and tungsten silicide have been proposed. A metal silicide is typically formed by depositing the metal into the contact windows and reacting with the silicon at an elevated temperature.
Of the above silicides, platinum silicide (PtSi) has been particularly suitable because it is easy to form and extremely stable. Schottky diodes having PtSi contacts have near-ideal forward and reverse I-V characteristics.
Referring to the prior art on Ohmic contacts, U.S. Pat. No. 3,274,670 discloses forming low resistance electrical contacts to semiconductor devices by depositing on a silicon substrate a thin layer of platinum and heat treating to form platinum silicide. Thereafter, a composite layer of platinum, titanium and gold is deposited over the platinum silicide. U.S. Pat. No. 3,290,127 discloses forming platinum silicide as an active contact layer over the semiconductor. Silver-gold metallization is then formed over the contact layer. U.S. Pat. No. 3,893,160 discloses a resistive connecting contact for a silicon substrate formed from a layer sequence of platinum silicide-titanium-molybdenum-gold.
Referring to the prior art on Schottky barrier contacts, it is known from U.S. Pat. No. 3,906,540 to form on a silicon body a platinum silicide layer, then applying a refractory metal barrier such as molybdenum, titanium, tungsten, tantalum, followed by forming an aluminum contact layer. The refractory metal barrier prevents intra-diffusion of aluminum and silicon constituents during subsequent heat treatments. U.S. Pat. No. 3,995,301 discloses an Al.sub.2 Pt Schottky barrier contact structure obtained by first forming platinum silicide on a silicon substrate, applying an aluminum layer thereon followed by sintering.
The above discussed refractory metals are deposited by sputtering or electron beam evaporation, the former being the most common method. The sputtering systems disclosed in the articles entitled "Micro-structural and Electrical Properties of Thin PtSi films and Their Relationships to Deposition Parameters" by R.M. Anderson et al, Journal of the Electrochemical Society, Vol. 122, No. 7, pp. 1337-1347, July 1975 and "3.5 Morphologies of RF Sputter-deposited Solid Lubricants" by K. A. B. Andersson et al, Vacuum, Vol. 27. No. 4, pp. 379-382, 1977 are representative of the conventional sputter apparatus used for depositing platinum. Typically, in a prior art sputtering apparatus the semiconductor wafers to be coated are arranged on a substrate holder plate mounted inside a processing chamber. The source of refractory metal is arranged as a target plate inside the chamber. A positive pressure of an inert gas such as argon or nitrogen is maintained in the chamber. A suitable radio frequency (RF) is applied between the target plate and the substrate holder plate making them the anode and cathode, respectively. By high energy collision, the gas ions will eject particles from the target which traverse the chamber and deposit on the wafers.
To convert the deposited refractory metal into its corresponding silicide, the wafers are heated to the necessary temperature at which the metal-silicon reaction takes place. For example, for PtSi formation the temperature should be at least about 350.degree. C. Different techniques for heating the wafers have been proposed in the prior art. One such technique, which is disclosed in the article by R. M. Anderson et al, supra, is by contacting the substrate holder plate with a copper block having an electrical heater element embedded therein. Another technique, which is disclosed in U.S. Pat. No. 5,545,115 by H. J. Bauer et al and assigned to the present assignee, is by means of a heating coil arranged at a distance behind the holder plate. In both these techniques, since the wafers are not heated directly, but instead the wafer holder plate is first heated, followed by thermal conduction of the heat from the holder plate to the wafers, inordinately long times will be necessary to form the silicide. Such long times are unsuitable for high volume manufacturing of integrated circuit contacts. Another disadvantage of this prior art technique is nonuniform heating of the wafers due to nonplanarity associated with the mutually contacting surfaces of the wafers and the holder plate. In other words, due to inherent wafer warpage and the holder plate surface being not perfectly flat, significant gaps exist between the wafer and the holder plate surfaces rendering heat transfer from the plate to the wafer portions corresponding to the gap significantly less than that to the remaining portions. As a result, "cold zones" are established in the wafer which would inhibit formation of the silicide contacts therein.
Another prior art technique for heating the wafers is by utilizing high intensity lamps mounted inside the processing chamber either behind or in front of the wafer holder plate. When the lamps are mounted behind the plate, the disadvantages enumerated hereinabove will heating coils will result. When the lamps are mounted in the front, they tend to be ineffective to rise the temperature of the wafers to the silicide-forming temperature since the first, thinnest platinum layer deposited on the wafer, due to its high reflectivity, tends to reflect off the light. Another disadvantage with lamp heater is that during the sputtering process the platinum will deposit on the lamp surface rendering the lamp to be an ineffective heat source. Yet another disadvantage with lamp heater is that it tends to introduce alkali contaminants such as sodium present in the glass casing of the lamp into the contact areas degrading the contact characteristics. Another disadvantage is that since a lamp heater radiates heat to the entire process chamber including the chamber walls and fixtures, water vapor, oxygen, etc. are released which tend to be incorporated into the wafers, typically in the form of a thin native silicon dioxide layer in the contact openings, rendering the silicide contacts of poor quality.
Accordingly, it is an object of the invention to provide a sputtering apparatus which allows rapid heating of the wafers for forming metal silicide thereon.
It is a further object of the invention to provide a sputtering apparatus which permits sputter cleaning of the wafer contact windows prior to metal deposition and conversion to metal silicide following the metal deposition.
It is still another object of the invention to provide a reliable method of forming high quality metal silicide contacts to a silicon workpiece.